Most escalators run continuously at a fixed speed. This, of course, wastes energy when there is no traffic. One solution is to start the escalator only when a passenger approaches. However, frequent starting of the escalator greatly increases mechanical wear. Furthermore, passengers may shy away from a stopped escalator, believing it to be out of service. Another solution is to provide two speeds of operation; a high speed for transporting passengers, and a lower, idle speed when the escalator is unoccupied. Such a concept is disclosed, for instance, in the context of DC motor control, in commonly-owned U.K. Patent Application No. G.B. 2,050,984A. The requirement to influence the quantity of passengers moved by an escalator, particularly in public transport systems, such as undergrounds or other railway systems, is another problem.
It is known to vary the speed of an AC motor by reducing the voltage supplied thereto with an AC chopper. Since the torque of the motor is proportional to the square of the voltage, speed can indirectly be influenced this way. However, this technique for motor speed control suffers from high motor losses and thus low efficiency at low speeds. This is because the speed is controlled by means of changing the slip, what is the only way when having a fixed motor frequency. Furthermore, in order to provide regulated motor speed throughout the entire speed range, additional regulating devices are required.